Laser fabrication of ceramic parts

ABSTRACT

A method of fabricating ceramic parts is disclosed using a laser deposition process to produce highly dense ceramic parts. A metal substrate is preferably used and ceramic powder deposited in layers on the substrate, while varying the power of the laser beam to bond the layers together without cracking the substrate or causing a plasma reaction in the ceramic powder. Dense structures which are about 96% to about 100% ceramic can be produced. A part can be graded using different types and mixtures of ceramic powders to produce the part of a desired composition.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of International Application PCT/US00/31675 filed on Nov. 16, 2000, which designated the United States and claimed the priority of U.S. Provisional Application No. 60/165,658 filed on Nov. 16, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with U.S. Government support under Contract No. F29601-00-C-0189 monitored by the Department of the Air Force and funded by the Ballistic Missile Defense Organization. The Government has certain rights in the invention.

FIELD OF INVENTION

[0003] The present invention resides in the field of laser fabrication of parts, and more particularly relates to methods of shaping ceramic powder materials to form such parts.

BACKGROUND OF THE INVENTION

[0004] Conventional methods of producing parts from metals, metal alloys, and/or ceramics include additive and subtractive methods. Subtractive methods teach subtracting material from a starting block to produce a more complex shape. Examples include milling, grinding, drilling, and lathe cutting. Conventional subtractive methods are deficient because they produce a large amount of waste material for disposal, involve a large initial expense in setting up the tooling, and result in significant tool wear which increases operating costs. Furthermore, such methods cannot produce parts with unique shapes or complicated internal formations.

[0005] Other conventional processes are additive, such as welding, plating, and cladding. Such processes are generally limited to coating or depositing material on a starting article. More recently lasers have been used to build a part under computer aided design/computer aided manufacturing (CAD/CAM) control, where a laser is directed at a surface and powder flows to the surface from a hopper via a powder feed device. The laser builds the part in layers as it heats, bonds, and shapes the powder in a desired configuration. Such a method is disclosed in U.S. Pat. No. 5,156,697 to Bourell et al. (Bourell). Bourell is hereby incorporated by reference into the present application.

[0006] Bourell is directed to the production of parts from (1) powders mixed together, such as a mixture of fluorophosphate glass powders and alumina powders; and (2) coated ceramic powders, such as aluminum silicate or silica coated with a polymer (see Column 8, lines 45-60). In the above arrangements, a polymer or wax-like substance is used to bond together the layers of laser-deposited particles. Such a bonding layer is required because the ceramic particles are heated to a temperature below their melting point, and thus the wax is required to bond them together.

[0007] The selective sintering process of Bourell produces a ceramic part having a density of about 40% to about 65%. After selective sintering, a further process/step is required to remove the bonding layer (polymer/wax)—otherwise the part would remain soft and unstable. The further processing step comprises placing the ceramic part in a furnace for post-deposition heating to remove the bonding layer. As the bonding layer is removed, the ceramic particles bond to themselves, but the particles are not melted together.

[0008] Ceramic parts having a high density and good mechanical properties have not heretofore been produced using a single-step laser deposition method.

SUMMARY OF THE INVENTION

[0009] It has been unexpectedly found that articles of manufacture, made from one or more ceramics, having a near net shape, good mechanical properties, and a high density could be produced in accordance with the teachings of the present invention. Previously known laser deposition methods produced ceramic parts with a density on the order of about 40% to about 65% in the ceramic part. Such methods required the use of a wax or other material to bond the layers together and a subsequent process or step to remove the wax/bonding layer. Thus, known laser processing methods have not been heretofore produced a sufficiently dense ceramic part in a single processing step, because a wax or similar substance was required to bond the ceramic particles together, and subsequent removal of the bonding agent required at least one further processing step.

[0010] According to the present invention, a novel method and articles (e.g. parts and structures) obtained by the method are disclosed, where a high density ceramic part is produced by melting together ceramic particles without the use of a bonding agent. The laser power is varied to allow sufficient bonding of the ceramic layer(s) together and to a substrate. The power is controlled in order to prevent plasma reactions which can occur in ceramic powders and cracking of the solid ceramic part. The method, employing a laser deposition process, builds the part layer-by-layer in a predetermined configuration.

[0011] The present invention utilizes the Laser Engineered Net Shaping (LENS) process, in conjunction with one or more ceramic powders, for direct fabrication of a high density ceramic structure or part. The ceramic part is formed by transporting one or more ceramic materials in powder form to a laser device where the ceramic powder(s) are melted and deposited on a substrate to form a ceramic part of high density. The density of the ceramic part can be greater than about 90% to about 100%, and is preferably from about 96% to about 100% dense. Such a high density is achieved according to a method which provides for varying the power of the laser beam during the deposition process. Thus, a highly dense ceramic part can be produced by a single laser deposition process, without requiring further steps such as post-deposition heating of the ceramic part in a furnace.

[0012] According to one method for use in the present invention, a substrate made of a metal, a metal alloy, or a ceramic serves as a base to support the formation of one or more layers of ceramic materials. The ceramic layers are formed by a laser deposition process, where one or more ceramic powders are fed from hopper(s) toward the substrate. A laser beam is directed at the substrate and the powder feed such that ceramic powder is deposited in the desired shape on the substrate. Successive layers of ceramic material are built upon the substrate and the laser beam is adjusted under CAD/CAM control to form a near net shape part in the desired shape. Custom designed cellular structures and complex geometric shapes can be formed.

[0013] According to the present invention, the laser power can be varied during the laser deposition process, in order to create a part having high density in which successive layers bond to one another and the substrate. For example, where the substrate comprises a metal or a metal alloy, the laser power is initially set at a high level to create a melt pool. As ceramic powder is deposited, the laser power is gradually reduced so that the powder melts but does not undergo a plasma reaction. The absorptivity of ceramic materials is significantly higher than the metal substrate, and thus reduced heat (and a lower laser power) is required to melt and bond the particles to form layers. The laser power can be selectively varied during the various steps of the laser deposition process.

[0014] In certain embodiments of the present invention, multiple ceramic powder feeds can be employed to create a hybrid ceramic part. A plurality of hoppers can feed different types of ceramic particles, and the hoppers can be selectively controlled to feed an appropriate amount of one or more types of ceramic powders. The multiple feeds can be selectively graded to form a hybrid part having different regions corresponding to different ceramic powders.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference characters denote corresponding parts throughout the several views and wherein:

[0016]FIG. 1 is a schematic representation of a laser fabrication process used to form ceramic parts according to the present invention;

[0017]FIG. 2 is an example of a failed ceramic part construction, where a ceramic substrate and ceramic layers were formed at a constant, high laser power;

[0018]FIG. 3 is a solid, flat specimen made of Al₂O₃ according to the method of the present invention;

[0019]FIG. 4 is a solid, tensile specimen made of Al₂O₃ according to the method of the present invention;

[0020]FIG. 5 is a hollow, flat specimen with a thin wall made of Al₂O₃ according to the method of the present invention;

[0021]FIG. 6 is a hybrid ceramic structure made of 50% Al₂O₃ and 50% AlN according to the method of the present invention;

[0022]FIG. 7 is a schematic representation of a hybrid ceramic part graded from 100% Al₂O₃ gradually up to 100% AlN made according to the method of the present invention;

[0023]FIG. 8 is a hybrid ceramic part made according to the specifications of FIG. 7;

[0024] FIGS. 9(a) and 9(b) are schematic representations of two steps in a method of joining a ceramic deposited material to a hybrid substrate, using the laser deposition process;

[0025]FIG. 10 is a schematic representation of a particular arrangement for grading ceramic materials in a saw-tooth formation using the laser deposition process; and

[0026]FIG. 11 is a schematic representation of another arrangement for grading ceramic materials using the laser deposition process.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERRED EMBODIMENT(S)

[0027] Ceramic parts and structures with high density and good mechanical properties are provided according to a method of the present invention. One preferred method of forming the ceramic parts and structures is illustrated in FIG. 1. In this embodiment, the method of the present invention is carried out using a laser fabrication apparatus 10 having a first hopper 12 containing ceramic powder 14, where the first hopper deposits the powder 14 in a feed zone 24 beneath a laser 16. A laser beam 18 is directed at the feed zone to heat and melt the powder to form a deposition in a structure or part 26. Material is added to previous depositions formed upon a substrate 30 to gradually build the structure/part. The substrate can be preheated before receiving depositions of the structure/part, but preheating is not required according to the present invention.

[0028] According to one preferred embodiment of the invention, the powder 14 comprises particles which are preferably of approximately uniform size and consisting of one type of ceramic. The appropriate size of the particles can be determined by one skilled in the art for the materials and equipment used in the laser deposition process. Alternatively, different ceramic powders can be mixed in the first hopper 12. Ceramic powders which are preferred for use in the present invention include aluminum oxide Al₂O₃ (also known as “alumina”) and aluminum nitride AlN. Other ceramics which can be used in the present invention are various oxides and non-oxides such as carbides, nitrides, and borides. Examples of these ceramics include, but are not limited to the following: ZrO₂, TiO₂, MgO, SiC, B₄C, BN, SiO₂, Si₃N₄, WC, TiC, TiB, TiB₂, TiN, 3Al₂O₃-2SiO₂, and MgO-Al₂O₃.

[0029] According to the present invention, the substrate 30 can be made of a metal, a metal alloy, or a ceramic. The substrate can include a hybrid of one or more of these materials. For example, the substrate can be arranged such that one portion of the substrate comprises Material A and another portion of the substrate comprises Material B (see FIG. 9(a)).

[0030] The laser 16 is preferably used in the Laser Engineered Net Shaping (LENS) process. The laser is guided under computer control to follow a predetermined pattern to build the structure or part. The laser is operated using computer aided design/computer aided manufacture (CAD/CAM) techniques, where the two-dimensional plane of the substrate 30 contains imaginary x- and y-axes, and the laser moves longitudinally toward and away from the substrate along an imaginary z-axis under control of a computer 28.

[0031] Laser fabrication provides for rapid cooling, as the laser beam 18 produces intense heat directed at relatively local regions of the deposited material. Due to the relatively large surface area of the melted material compared to its volume, energy can be removed rapidly and fast cooling is achieved. Fine grain structure is achieved in the part due to the rapid cooling. Because near net shape processing is obtained, the part requires minimal post machining. In fact, no additional steps or processes are required to produce a highly dense part.

[0032] According to a preferred method for producing ceramic parts and structures according to the present invention, the substrate 30 is preferably made of a metal or a metal alloy. For example, the substrate can comprise a well known titanium based alloy such as Ti-6Al-4V, also known as Ti64. When a metal substrate is used, the laser power is initially set sufficiently high to melt the metal material at the surface of the substrate, thereby creating a melt pool. Thereafter, a laser deposition process (e.g. the LENS process) is initiated. Ceramic powder 14 is dispensed through the hopper 12 in a controlled manner. As the powder is deposited in the feed area 24, the laser 14 is guided under computer control to heat and shape the powder in one or more layers, thereby building the ceramic part 26.

[0033] During the laser deposition process, the laser power is varied and thus the amount of heat directed at the substrate and ceramic powder can be controlled. As discussed above, the laser power is initially set at a high level, generally in a range of about 150 W to about 550 W, at a level which can readily be determined by one skilled in the art to melt the substrate to thereby create a melt pool. It is important that a melt pool be created, in order for the ceramic material to bond with the substrate material. As ceramic powder flows into the feed area 24, the ceramic material forms a layer of ceramic material and bonds with the substrate. Successive layers of ceramic material are deposited on each other and the substrate. During the deposition process, the laser power is gradually reduced from the initially high level required to melt the metal substrate to a lower level for melting the ceramic material. The absorptivity of ceramic materials is significantly higher than the metal substrate, so a reduced laser power is required to melt the ceramic powder without vaporizing it.

[0034] If a ceramic substrate is used, preferably the ceramic substrate is heated to a temperature sufficient to melt the substrate, and then the laser power is adjusted to an appropriate level to melt the ceramic powder as it is deposited.

[0035] According to this method, the laser power is reduced during deposition of the ceramic powder, to a level suitable for the type of ceramic powder to be deposited. The laser power should be set at a sufficient level to melt the ceramic powder without causing damage to the ceramic material. When a high powered laser beam is directed at the ceramic powder, the powder turns a deep, dark color, indicating a plasma reaction has occurred which results in material evaporation and a porous structure. Such a reaction can be avoided by setting the laser power at a level only high enough to melt the ceramic powder to allow the ceramic powder to bond together.

[0036]FIG. 2 illustrates an example of problems which can be encountered when attempting to use a laser deposition process to build a ceramic part upon a ceramic substrate. In the example shown, a ceramic substrate made of Al₂O₃ was used. The substrate was preheated to 500° F. using a hot plate, in order to help prevent cracking of the substrate. However, when a high laser power (375 W) was incident on the substrate, the result was thermal shock due to the focus of a high powered laser on a portion of the solid ceramic structure. The thermal shock caused cracking of the ceramic substrate. Thereafter, a ceramic powder made of Al₂O₃ was fed to the substrate using a laser deposition process. As the laser power was reduced to 130 W, the substrate remained intact, but the deposited material turned to a dark color (black), indicating a plasma reaction had occurred. When the laser power was further reduced to 60 W, the deposited material appeared less dark. However, for ceramic material deposited at a laser power of 60 W, the deposited layers peeled off the substrate, as there was insufficient bonding between the ceramic substrate and the deposited ceramic material.

[0037] In general, in the methods of the present invention, both the high laser power required to initially create a melt pool on the substrate, and the lower laser power preferred to avoid a plasma reaction in the deposited ceramic material are taken into consideration in selecting reaction parameters. A minimum laser power of approximately 30 W is typically required for the laser fabrication device described with reference to FIG. 1 to function properly.

[0038] A solution to the above-described problems is to initially set the laser power at a high level and gradually reduce the power as the deposited ceramic material bonds to the substrate. According to one method for carrying out the present invention, a metallic substrate (Ti-6Al-4V alloy, also known as “Ti64”) was selected. A minimum power of about 150 W is required to melt the Ti64, which is beyond the acceptable range of laser power for heating the ceramic powder. In this method, the laser was initially set at about 250 W to melt the Ti64 surface, and the power was gradually reduced to about 60 W during ceramic powder deposition. Parts fabricated according to this method are shown in FIGS. 3-5. FIG. 3 is a photograph of a solid, flat bar which was fabricated from Al₂O₃ ceramic powder. FIG. 4 is a photograph of a solid, tensile cylindrical bar also fabricated from Al₂O₃ ceramic powder. The part depicted in FIG. 4 shows that the laser deposition process can be used to produce parts with complex geometric shapes. Parts with delicate geometries can also be produced, as shown in FIG. 5. FIG. 5 is a hollow, flat specimen made from Al₂O₃ ceramic powder, the final part having external dimensions of 3 in.×0.5 in.×0.08 in. with a thin wall less than 1 mm thick prepared using the laser deposition process.

[0039] The density of the above fabricated parts was measured using the standard water absorption method (ASTM designation: C373-56) well known in the art for obtaining accurate and reliable density calculations. The average density of the above fabricated Al₂O₃ parts is 3.80 g·cm³ which indicates 0.962 densification of the theoretical density of Al₂O₃ (3.95 g·cm³). Thus, the finished ceramic parts were, on average, about 96% dense.

[0040] The density of parts or structures fabricated according to the present invention are preferably greater than about 90%, and are up to about 100% dense, but preferably in the range of from about 96% to about 100% dense. The high density of ceramic parts produced according to this method (as compared with prior art laser deposition methods which generally achieve only about 40% to about 65% dense ceramic parts, unless, e.g., a further heating step in a furnace is undertaken) is achieved by using a metallic substrate and varying the power of the laser beam as discussed herein.

[0041] Ceramic powder comprising Al₂O₃ is useful with the present invention, because Al₂O₃ powders generally have good fluidity and are commonly used for a variety of applications. While Al₂O₃ is one preferred type of ceramic powder suitable for use in the present invention, other types of ceramic powders can also be used, either alone or in conjunction with Al₂O₃. Another preferred ceramic powder is AlN, which often includes some fine particulates which tend to clog up the hoppers and feed system. Thus, when using AlN powder, the powder can be filtered and treated with a sufficient amount of fluid and dispersant to remove the fine particles. The remaining powder has a much better fluidity and can be successfully used in the laser fabrication device.

[0042] Referring back to FIG. 1, a multiple feed laser fabrication apparatus is shown, where the apparatus includes a second hopper 20 containing a second ceramic powder 22 which can be controllably released toward the feed area 24. The second hopper can contain the same type of ceramic powder as the first hopper 12, in order to allow for continuous part fabrication when building, e.g., a large part or a plurality of uniform parts. The second hopper 20 can also be used to store particles corresponding to a different type of ceramic powder from that stored in the first hopper 12.

[0043] When filling the hoppers 12 and 20 with different types of ceramic powder, feeding can be accomplished by controlling the hoppers to selectively release one or both types of particles at predetermined times. By controlling the first and second hoppers, a graded ceramic part can be produced. FIG. 6 is a photograph of a part produced by this method. As shown in FIG. 6, approximately the first one-half of the length of the part was fabricated using Al₂O₃ powder; the other half was fabricated using a dual feed mixture of about 50% Al₂O₃ and about 50% AlN. As described herein, the graded structures are made from powders with approximately the stated percentage of the material. It is possible that a small amount of impurities can be present in the finished part.

[0044] Accordingly, in the above example, the first hopper 12 contained Al₂O₃ powder and the second hopper 20 contained AlN powder. Approximately one-half of the fabrication process involved feeding exclusively from the first hopper, while approximately the other half was produced with simultaneous feeding from the first and second hoppers. The result was a solid graded part with no visible defects and a smooth interface between the 100% Al₂O₃ and the 50% Al₂O₃/50% AlN sections.

[0045] More complex variations of the above graded part have been produced. For example, FIG. 7 depicts a specification for a part graded from 100% Al₂O₃ gradually to 100% AlN. As shown in FIG. 7, the specification calls for a 100% Al₂O₃ section, followed by a 50% Al₂O₃/50% AlN section, and then a 25% Al₂O₃/75% AlN section, where each of these sections is produced using a laser power of 125 W. The next section is 100% AlN produced under a laser power of 14-5 W. The change in laser power for the AlN section is motivated by the different physical properties of AlN. Aluminum nitride has a higher thermal conductivity and a lower laser energy absorption than alumina. Accordingly, a higher laser power is required to melt and bond together the layers of aluminum nitride. FIG. 8 is a photograph of a graded ceramic part produced according to the specification of FIG. 7. As seen in FIG. 8, the transitions between the various graded sections are smooth. Only the 25% Al₂O₃/75% AlN experienced some bonding problems, which was likely due to insufficient laser power caused by the higher percentage of AlN as compared to the other sections having Al₂O₃. The laser power can be varied according to the present invention to allow for an increase in laser power for the 25% Al₂O₃/75% AlN section, where the applied laser power would be in the range of approximately 125 W-145 W.

[0046] Variations in the method of the present invention include feeding different types of ceramic powders (as described above) and starting with a substrate made up of different materials. FIGS. 9(a) and 9(b) illustrate two steps in a process of depositing a ceramic material on a hybrid substrate. The starting substrate material comprises “Material A” and “Material B” which can be metals, metal alloys, or ceramic materials. Metals or metal alloys are preferred, in order to provide sufficient bonding with the deposited ceramic material, for reasons stated above. The “deposited material” can be a ceramic material deposited by the aforementioned laser deposition process. The ceramic material can also comprise a graded ceramic structure.

[0047] As shown in FIGS. 10 and 11, the ceramic materials graded in a structure or part can be deposited in a variety of formations. FIG. 10 illustrates a saw tooth formation with “Ceramic C” on one side and “Ceramic D” on the other side of the structure. Alternatively, as shown in FIG. 11, the grading can be arranged using interlocking teeth, with “Ceramic E” on one side and “Ceramic F” on another side of the structure. The method and article produced according to the present invention are not limited to the above-mentioned graded parts or structures.

[0048] Although the invention has been described in detail including the preferred embodiments thereof, such description is for illustrative purposes only, and it is to be understood that changes and variations including improvements may be made by those skilled in the art without departing from the spirit or scope of the following claims. 

What is claimed is:
 1. A method of constructing a ceramic part having a high density, the method comprising the steps of: providing a substrate made of a metal, a metal alloy, or a ceramic; depositing a ceramic powder on the substrate; directing a laser at the substrate to heat the substrate and melt the powder during the depositing step, thereby shaping the powder as the laser follows a predetermined pattern over the powder and substrate; and varying the power of the laser during the directing step to allow sufficient melting to form a highly dense structure.
 2. The method of claim 1, wherein the highly dense structure is formed substantially without cracking or a plasma reaction in the powder.
 3. The method of claim 1, and further comprising repeating the depositing step to form multiple layers from the ceramic powder.
 4. The method of claim 1, wherein the density of the ceramic part is from about 90% to about 100% dense.
 5. The method of claim 1, wherein the density of the ceramic part is from about 96% to about 100% dense.
 6. The method of claim 1, wherein the ceramic powder comprises an oxide or a non-oxide ceramic powder.
 7. The method of claim 1, wherein the ceramic powder comprises Al₂O₃.
 8. The method of claim 1, wherein the ceramic powder comprises AlN.
 9. The method of claim 1, wherein the substrate includes Ti-6Al-4V.
 10. The method of claim 1, wherein the step of varying the power is performed over a power range of about 30 W to about 550 W.
 11. The method of claim 1, wherein the step of varying the power is performed over a power range of about 30 W to about 150 W.
 12. The method of claim 1, wherein the step of varying the power comprises reducing the laser power from about 150 W to about 60 W.
 13. The method of claim 1, wherein the depositing step further comprises separately feeding first and second types of ceramic particles.
 14. The method of claim 13, wherein the depositing step further comprises selectively forming layers of the first and second types of ceramic particles on the part.
 15. An article of manufacture comprising a ceramic part having a high density obtainable by a method comprising the steps of: providing a substrate made of a metal, a metal alloy, or a ceramic; depositing a ceramic powder on the substrate; directing a laser at the substrate to heat the substrate and melt the powder during the depositing step, thereby shaping the powder as the laser follows a predetermined pattern over the powder and substrate; and varying the power of the laser during the directing step to allow sufficient melting to form the highly dense ceramic part.
 16. The article of claim 15, wherein the highly dense part is formed substantially without cracking or a plasma reaction in the powder.
 17. The article of claim 15, and further comprising repeating the depositing step to form multiple layers from the ceramic powder.
 18. The article of claim 15, wherein the ceramic part is from about 90% to about 100% dense.
 19. The article of claim 15, wherein the density of the ceramic part is from about 96% to about 100% dense.
 20. The article of claim 15, wherein the ceramic powder comprises an oxide or a non-oxide ceramic powder.
 21. The article of claim 15, wherein the ceramic powder comprises Al₂O₃.
 22. The article of claim 15, wherein the ceramic powder comprises AlN.
 23. The article of claim 15, wherein the substrate includes Ti-6Al-4V.
 24. The article of claim 15, wherein the step of varying the power is performed over a power range of about 30 W to about 550 W.
 25. The article of claim 15, wherein the step of varying the power is performed over a power range of about 30 W to about 150 W.
 26. The article of claim 15, wherein the step of varying the power comprises reducing the laser power from about 150 W to about 60 W.
 27. The article of claim 15, wherein the depositing step further comprises separately feeding first and second types of ceramic particles.
 28. The article of claim 27, wherein the depositing step further comprises selectively forming layers of the first and second types of ceramic particles on the part.
 29. A method of constructing a ceramic part having a high density, the method comprising the steps of: providing a substrate made of a metal, a metal alloy, or a ceramic; depositing a ceramic powder on the substrate in a first layer; directing a laser at the substrate to heat and melt the ceramic powder during the depositing step, thereby shaping the powder as the laser follows a predetermined pattern over the powder and substrate; varying the power of the laser during the directing step to allow sufficient melting to form a highly dense structure; and forming at least one additional layer by depositing ceramic powder on the first layer.
 30. The method of claim 29, wherein the highly dense part is formed substantially without cracking or a plasma reaction in the powder.
 31. The article of claim 29, wherein the ceramic part is from about 90% to about 100% dense.
 32. The article of claim 29, wherein the density of the ceramic part is from about 96% to about 100% dense. 